A variable resistance memory with two terminals has been developed as a large-capacity non-volatile memory which may replace a floating-gate type NAND flash memory in the future. This type of memory makes it possible to achieve a low-voltage and low-current operation and a high-speed switching, to scale down a memory cell size, and to improve the integrity of memory cells.
Various materials have been proposed as the material forming a variable resistance layer of the variable resistance memory. For example, a stacked ReRAM in which a plurality of stacked high-permittivity insulating films are used as a variable resistance layer has good prospects in terms of, for example, compatibility with the existing LSI process and a low-current operation. In the stacked ReRAM, the memory is turned on and off by the movement of oxygen vacancies in the variable resistance layer.
In a large-capacity memory array, a plurality of metal lines which are called bit lines and word lines are arranged so as to intersect each other and memory cells are formed at the intersections between the bit lines and the word lines. Data is written to one memory cell by applying a voltage to the bit line and the word line connected to the memory cell.
A plurality of memory cells, for example, tens to thousands of memory cells are connected to one bit line and one word line. Therefore, during a write operation, a voltage (a half-select voltage: a voltage lower than the voltage applied to a selected cell) is applied to a plurality of cells connected to the same bit line and word line as those connected to a write target cell (selected cell) and a current (half-select leakage current) flows to the cells. When the amount of half-select leakage current increases, the power consumption of a memory device increases. And when the amount of half-select leakage current increases, because a voltage drop in the bit lines and word lines increases, a sufficiently high voltage cannot be applied to the selected cell. For this reason, it is necessary to achieve a memory cell with a small amount of half-select leakage current in order to form a large-capacity memory array.